Chrome-magnesia refractories



Patented Oct. 5, 1954 UNITED STATS ATENT QFFICE 2,690,974 CHROME-MAGNESIA asrnec'rortms No Drawing. ApplicationApi-il 16, 1952, Serial No. 282,708

9 Claims.

This invention relates to fused or heat-cast refractories and is particularly concerned with the provision of an improved refractory especially adapted for use in the manufacture of steel.

In my Patent No. 2,599,566, issued June 10, 1952 on my copending application Serial No. 233,945, filed June 27, 1951, as a continuationin-part of my prior applications Serial No. 7,710, filed February 11, 1948, and now abandoned, and SerialNo. 83,999, filed March 28, 1949, and also now abandoned, I have described a dense, fused high-iron chrome-spinel refractory that possesses marked resistance to heat shock or spalling together with good resistance to attack by ferruginous slags and is thus of unusual value in the construction of open hearth furnaces. Such refractory contains FeO, MgO, A1203 and CI'zOs and is particularly characterized by a magnesia content sufficient to form a separate periclase phase in substantial amount and by a ratio of the mole of R0 oxides to the mols of R203 oxides of at least 2.2 and advantageously at least 3.5, with such molal ratio being at least 4.0 for best resistance to spalling and to slag attack. The FeO content of such refractory is partitioned between the spinel phase and the periclase phase and ranges from 5 to 25% and preferably falls between '7 and 20%, with such range being desirably s to 17% when the molal ratio of ac oxides to R203 oxides is at least 4.0.

During use in an open hearth furnace, for example, whether located at the slag line or above, thisfused chrome-spinelrefractory does undergo gradual corrosion but at an appreciably slower rate than comparable burnt chrome and magnesite refractories. What happens, as has been determined by analytical examination and observation, is that the refractory absorbs iron in the form of F6203 at its surface, which absorbed iron combines with magnesia from the periclase phase to form magnesio-ferrite, MgQFezOa. As the amount of this magnesio ferrite, which has a melting point only slightly above the usual operating temperature of an open hearth furnace and which forms a solid solution with the chromecontaining spinel, increases, the melting point of the refractory at such surface and hence its refractoriness are correspondingly lowered, with the result that the refractory gradually disappears through fiuxing and liquefaction. Where the refractory is in contact with molten slag, this fluxing action is of course accentuated by the eroding effect of the slag itself.

It would appear, at least theoretically, that the corrosion-erosion resistance of this fused re- (Cl. ltd-59) fractory could be improved in this respect by materially decreasing it's FeO content. There would then be less iron available in the refrac tory itself for conversion to Fezos during use with concomitant formation of MgOFezOs, and the life of the refractory should be correspondingly extended.

I have now discovered that the corrosion-ew sion resistance of such fused chrome spinel refractory can be increased by so decreasing its initial FeO content, without adversely affecting its spalling resistance, provided that a satisfactcry molal ratio of the R0 oxides to the R203 oxides is maintained. Such ratio, I have found, should be at least 2.4 and is advantageouslyat least 3.5. I have also discovered that, surpris inely, with an Feo content of less than 5%. resistance to spelling becomes again poor if this ratio exceeds about 10; and such ratio should desirably be no greater than 9.0. I

As with the fused refractories describediii my cope'hdiiig application Serial No. 233,945, the coefficient or expansion or the presentrusea refractorles is not high relative to that of other refractories and sc'r'ne instances is even less than the expansion coefficients of various coininercial burnt chrome and rhagnesite refractories. The tensile strength of the present refractories is also higher than that of commercial burnt chrome and magnesite refractories so that greater strains can be developed therein before failure. On the other hand the thermal conductivity of such refractories is also higher 'so that the teniperature gradients obtained within such refractori'es when they are subjected to alternate heat ing and cooling are lessened. Apparently these properties are inore'significant with respect to spallin'gresistance than the dense structure of these refractories. V

As will be appreciated, relatively high temperatures are required to fuse the present composi tions so that electric melting must be resorted to. The raw materials can be melted in a conventional electric furnace, the shell of which is lined with materiai of a composition similar to that of the refractory to be produced. The molten material can then be castinto molds. The resulting refractory pieces, following initial solidification, should be annealed either from their o n heat of crystallization with the aid of appropriate insulation 01'' with the addition of external heat in alehr. v v

In the resulting fused refractory, as with the refractory described in y cop-ending application, the Eco is partitioned between the periclase phase, in which it is present in solid solution, and the spinel phase, in which it is present, of course, as an R spinel-forming oxide. Apparently what happens is that, as the spinel phase is formed upon cooling and solidification of the molten refractory mass, the MgO forms spinels with the R203 oxides in preference to the FeO so that, in effect, the FeO is in part displaced into the periclase phase formed by the remaining MgO. So far as can be determined, this partitioning effect occurs even in those refractory compositions containing the smallest amounts of FeO within the range set out below.

The proportions within which the several oxides forming the basis of the present refractory can be varied are determined in part by the effect of the variation on the spalling resistance of the refractory. The A1203 and CrzOs content, for example, cannot be increased to such an extent that the ratio of the mols of RO oxides to the mols of R203 oxides falls below about 2.4 and the resistance of the refractory to spalling becomes unsatisfactory; neither can their content be so lowered that such ratio exceeds about and the spalling resistance again becomes unsatisfactory. For the same reason, the content of FeO and MgO cannot be so lowered that such ratio becomes less than 2.4, or so increased that such ratio exceeds about 10. Moreover, as indicated above, suflicient MgO must be present to form the periclase phase, which, as can be readily determined, will amount to a minimum of approximately 23%. As will be apparent, a periclase phase can be formed only if an excess of R0 oxide exists beyond that required to combine with the R: oxides to form RO.R203.

These proportions are also in part governed by the nature of the slag with which the refractory is to come in contact. While, for example, the resistance of the present refractory to attack by basic slag improves with an increase in MgO (and, hence, the periclase phase), its resistance to acid slag attack tends to correspondingly decrease. Conversely, too low an MgO content decreases the ability of the refractory to withstand basic slag attack. Similarly, an increase in CH0: (and, hence, the chrome-containing spinel phase) improves the resistance of the refractory to acid slag attack but tends to lower its resist ance to basic slag. Too low a CrzOa content, on the other hand, decreases the resistance of the refractory to acid slag attack. A high ratio of CrzOs to A1203 also facors acid slag resistance. Where both types of slag are encountered as in open hearth practice, a practical compromise is of course necessary.

Additionally, as the MgO content is increased, the temperature required to effect fusion also increases with the results that volatilization of MgO becomes more and more appreciable and the production of a commercially satisfactory and usable refractory casting becomes more and more difiicult.

Since the heat-casting process required for the manufacture of the present refractories is inherently more expensive than the conventional sintering techniques of making refractories, it is desirable that only commonly available raw ma terials such as calcined magnesite and chrome ore be used in the production of the instant refractories'. Calcined magnesite can be readily employed for such purpose and is relatively inexpensive. While chrome ore, which is basically chromite (FeO.CrzO3) but in which in nature appreciable MgO replaces FeO and appreciable A1203 replaces CrzOs in the spinel lattice, would be generally suitable for the present purpose if its FeO content were on the low side, such a lowiron chrome ore is unfortunately relatively exensive. A similar situation obtains with other low iron-content raw materials suitable for producing the instant refractory. It is, of course, entirely possible to provide the several oxides constituting the present refractory in more or less pure condition and to fuse an appropriate mixture thereof, but such procedure is obviously commercially impracticable.

I have found, nevertheless, that the less expensive, higher iron-content chrome ores can be utilized in the production of the present refractory by providing for elimination of the necessary amount of FeO from the refractory mass during its fusion. This purpose can be accomplished, for example, by effecting reduction of the FeO to metallic iron as through the use of metallic aluminum, which is desirably added to the molten refractory mass just prior to casting thereof. The resulting metallic iron settles to the bottom of the melting chamber. Since A1203 is also formed in such reaction, the amounts of the several raw materials should be suitably adjusted to provide the desired oxide proportions in the fused refractory. Reduction of the FeO content to less than about 1 however is not practicable in view of the excessive amount of aluminum that would be required, in which case excessive reduction of C12O3 to metallic chromium would also be encountered.

With these several considerations in mind, then, the present refractory composition comprises and analytically contains 1% but less than 5% FeO, 40 to 78% MgO, 5 to 45% A1203, and 5 to 50% CrzOz, with the ratio of the mols of FeO and MgO to the mols of A1203 and CrzOa being between 2.4 and 10. The total of such oxides should amount to at least 87% and advantageously at least 92% in order to provide satisfactory spalling resistance and resistance to slag attack.

As well known, commercial chrome ores contain a small amount of a siliceous gangue. When such a chrome ore is used as a raw material in the preparation of the present refractory, then, a siliceous matrix containing any impurities is also formed upon solidification of the molten refractory composition. This siliceous matrix is largely crystalline in nature, even in relatively rapidly cooled castings. In the absence of appreciable calcium oxide, this siliceous matrix or phase has the crystallographic properties of forsterite, 2MgO.SiO2, with presumably some FeO replacing a part of the MgO.

Moderate amounts of this siliceous matrix, such as are fortunately obtained with commercial grade chrome ores, are not harmful either to the heat-shock resistance of the instant refractory or to its resistance to acid slag attack. As the percentage of silica is increased, however, resistance to spalling decreases; and with larger amounts the castings crack so badly even before heating that they are unsaleable. Moreover, as the silica content is increased, the resistance of the present refractory against acid slag decreases. From this standpoint as well as the standpoint of heat-shock resistance, therefore, the SiOz content should be not more than 9% and advantageously not more than '7 With S102 in the composition, the periclase phase can, of course, be formed only if there is an excess of R0 oxide beyond that required to combine with the S102 to form 230.8102 and to combine with the A1202 and the Cr203 to form B03203. An excess of R0 oxide is therefore indicated when the ratio of the mols of FeO and MgO to the mole of A1202 and 1203 P half the mole of S102 exceeds unity. As previously pointed out, this ratio is between 2.4 and and advantageously between 3.5 and 9.0 in the present refractory.

Commercially available calcined magnesites of good quality generally contain. a small amount of 02.0 as an impurity. Dolomitic magnesites may also be utilized under certain conditions, but then 0210 in appreciable quantities will be included in the refractory composition. Since CaO does not form a spinel, it will appear in the siliceous matrix when it is present in the raw materials. As the 0220 increases in proportion, it progressively replaces MgO to give monticellite, dicalcium silicate, and finally tricalcium silicate. If the combining power of the S102 is exceeded, calcium chromite is formed with the result that serious cracking of the castings occurs during their manufacture. The basic slag resistance of the present refractory is also decreased as the 0:20 content is increased, especially in the presence of significant quantities of S102. For these reasons, then, the CaO content should. not be more than 6%; and advantageous- 1y no more than 3% Ca'O is present.

Since the effect of CaO is to displace MgO from the siliceous matr x, an excess of R0 oxide is indicated in such case when the ratio of the mols of FeO, Mg0, and CaO to the mole of A1203 and @202 plus half the mole of S102 (if present) exceeds unity. As already stated, this ratio is between 2.4 and 10 and advantageously between 3.5 and in the present refractory.

In the of a chrome ore in the production of a refractory composition low in C1203, not much iron oxide will be introduced with the chrome ore, the amount depending of course on the grade of chrome ore employed. It may not be necessary, then, to reduce any of the iron oxide to metallic iron, in which event, if the chrome ore is partially oxidized, any F6203 present therein 1 remain as such in part in the final refractory. As illustrative of the small amounts of Fe203 that may be expected under such circumstances, a refractory of the present type containing 2.6% FeO by analysis also conta ned 1.3% F6202 by analysis. Since, as already in icated, MgOFezOa forms during use of the present type of refractory in an open hearth furwith gradual fluxing of the refractory, it is desirable of course to maintain the initial Fe202 content as low as possible in order to prolong the life of the refractory.

The melt (casting) analyses given in Table I are illustrative of the composition of the present refractory (as is usual practice with chrome refractories, total analyzed Fe is calculated as FeO in this and the following tables) Table I Melt F MgO Al202 ClzOs s10. CaO

2. 2 30.3 43. 0 22. 7 1. 3 0. 5 1. 6 42. 0 29. 6 24. 6 1.8 0.4 2. 7 43. 2 32. 6 20. 0 1. 2 0. 3 a. 9 33. 2 33. 0 7.1 2. 4 0. 4 1.8 56. 0 20.2 20.5 1.1 0. 4 3. 5 55. 2 16.0 20.6 4. s 0. 4 4. 2 64.7 19. 9 2. 0 1. 6 0. 6 4. 0 69. 2 14. 7 9. 0 2. 5 0.6 3.1 71.0 11.7 11.6 2.0 0.6 3. 6 73. 6 s. 4 10. 6 3. 2 0. 6

Each of these refractory castings comprises a chrome-containing spinel, periclase, and a siliceous matrix. There is only one spinel phase, which contains both FeO and MgO as RO oxides and both CrzOs and A1202 (and any F6203) as R203 oxides in random distribution. Only melts III to IX possess satisfactory resistance to spelling, however, melts I, II and X exhibiting poor or unsatisfactory spallin resistance. As can readily be determined (in the manner explained above), the molal ratio of R0 oxides to R203 oxides in melts III to IX is between 2.4 and 10, whereas such ratio for melts I and II is below 2.4 and for melt X is above 10.

Especially good resistance to heat shock or spallin' as well as good resistance to both acid slag and basic slag are obtained when the present refractory composition comprises and analytically contains 1% but less than 5% Eco, 45 to 75% MgO, 5 to 38% Ala-D2, and 5 to 45% C1203, the total of such oxides amounting to at least 92%, the molal ratio of R0 oxides to R203 oxides (as determined in accordance with the aboveexplained procedures) being between 3.5 and 9.0. Any S102 or Ca-O that may be present should desirably be no more than 7% or 3% respectively. The melt (casting) analyses given in Table II are illustrative of this preferred range of compositions:

Table II Melt r20 MgO A1202 orzol S102 020 4.1 56. 0 29.1 s. 3 5.1 0. 4 3.1 62.8 12. 7 1s. 4 2. 5 0. 5 4.4 58.8 12.2 21.1 3.0 0.5 2.6 61. 0 14. 2 1s. a 3. o 0. 6 1. 0 62.6 7.8 23. 9 3. a 0. 5 2. 4 66.9 14. 4 9. 4 6.3 0. 6 3. 0 69. 7 s. 2 16. 4 2.1 0.6

The melt (casting) analyses given in Table III are illustrative of the effect of too much 8102;

Table I I 1 Melt F60 Mgo A1 03 CIzOa $102 08.0

3. 1 62. 8 l2. 7 18. 4 2. 5 O. 5 4. 1 56. 0 29. 1 5. 3 5. 1 0. 4 2. 4 66. 9 14. 4 9. 4 6. 3 0. 6 2. 8 47. 4 23. 7 18. 2 7. 5 O. 4 2. 9 59. 4 15. 3 10. 9 10. 9 O. 6

The spelling resistance of the first four melts was satisfactory, but melt XIX cracked so badly during annealing that it was unusable.

Table IV presents several melt (casting) analyses illustrating the effect of too much CaO:

Table IV Melt FeO MgO A1203 C1203 S10 C20 The first three melts possessed satisfactory resistance to spalling. Melt XXII, on the other hand, contained more 0210 than could combine with the S102 and the refractory cracked during annealing. Moreover, as the S102 approaches the upper end of its range, and the CaO content should be maintained such that the total of S102 and 02.0 is no greater than 13% in order to avoid. a tendency to crack during annealing, as ex- 4 hibited by melts XXIII and XXIV. Advantageously, the total of S102 and CaO should generally not greatly exceed 8% since otherwise a decreased resistance to slag attack results.

I claim:

1. A fused refractory material comprising a chrome-containing spinel and periclase, said refractory material analytically containing 1% but less than FeO, 40 to 78% MgO, 5 to 45% A1203, and 5 to 50 CrzOa, the total of such oxides amounting to at least 87%, the ratio of the mols of FeO and MgO to the mols of A1203 and Cr203 being between 2.4 and 10.

2. The fused refractory material as claimed in claim 1, in which the total of FeO, MgO, A1203, and CrzOa amounts to at least 92%.

3. A fused refractory material comprising a chrome-containing spinel, periclase, and a siliceous matrix, said refractory material analytically containing 1% but less than 5% FeO, 40 to 78% MgO, 5 to 45% A1203, and 5 to 50% Cra, the total of such oxides amounting to at least 87%, and Si02 in an amount up to 9%, the ratio of the mols of FeO and MgO to the mols of A1203 and Cr202 plus half the mols of SiOz being between 2.4 and 10.

4. The fused refractory material as claimed in claim 3, in which the total of FeO, MgO, A1203, and Cr203 amounts to at least 92%.

5. A fused refractory material comprising a chrome-containing spinel, periclase, and a siliceous matrix, said refractory material analytically containing 1% but less than 5% FeO, to 78% MgO, 5 to A1203, and 5 to CI203, the total of such oxides amounting to at least 87%, S102 in an amount up to 9%, and CaO in an amount up to 6%, the ratio of the mols of FeO, MgO, and CaO to the mols of A1203 and Cr2O3 plus half the mols of Si02 between 2.4 and 10.

6. The fused refractory material as claimed in claim 5, in which the total of FeO, MgO, A1203, and CIzOa amounts to at least 92%.

7. A fused refractory material comprising a chrome-containing spinel and periclase, said refractory material analytically containing 1% but less than 5% FeO, 45 to MgO, 5 to 38% A1203, and 5 to 45% Cr203, the total of such oxides amounting to at least 92%, the ratio of the mols of FeO and MgO to the mols of A1203 and CIzOa being between 3.5 and 9.0.

8. A fused refractory material comprising a chrome-containing spinel, periclase, and a siliceous matrix, said refractory material analytically containing 1% but less than 5% FeO, 45 to 75% MgO, 5 to 38% A1203, and 5 to 45% C12O3, the total of such oxides amounting to at least 92%, and S102 in an amount up to 7%, the ratio of the mols of FeO and MgO to the mols of A1203 and 01203 plus half the mols of Si02 being between 3.5 and 9.0.

9. A fused refractory material comprising a chrome-containing spinel, periclase, and a siliceous matrix, said refractory material analytically containing 1% but less than 5% Foo, 45 to 75% MgO, 5 to 38% A1203, and 5 to 45% @203, the total of such oxides amounting to at least 92%, Si02 in an amount up to 7%, and CaO in an amount up to 3 the ratio of the mols of FeO, MgO, and CaO to the mols of A1203 and. CrzOa plus half the mols of Si02 being between 3.5 and. 9.0.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,358,652 Nicholas Sept. 19, 1944 2,599,566 Magri June 10, 1952 

1. A FUSED REFRACTORY MATERIAL COMPRISING A CHROME-CONTAINING SPINEL AND PERICLASE, SAID REFRACTORY MATERIAL ANALYTICALLY CONTAINING 1% BUT LESS THAN 5% FEO, 40 TO 78% MGO, 5 TO 45% AL2O3, AND 5 TO 50% CR2O3, THE TOTAL OF SUCH OXIDES AMOUNTING TO AT LEAST 87%, THE RATIO OF THE MOLS OF FEO AND MGO TO THE MOLS OF AL2O3 AND CR2O3 BEING BETWEEN 2.4 AND
 10. 